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AMTA Paper Archive

Near-field Electromagnetic Holography in Conductive Media
Earl Williams, November 2009

This paper presents a new approach to the inversion of boundary value (BV) problems in an infinite conductive, homogeneous media. Our interest is to investigate the possibility of imaging underwater electromagnetic sources from remote electromagnetic sensor data. Specifically, given two polarizations of the electric/magnetic fields on a cylindrical surface exterior to the electric and magnetic sources, we develop a frequency domain, back-projection technique that allows for the complete determination of the electric and magnetic fields in the region between the BV surface and the sources. This is an inverse problem and Tikhonov regularization is used to obtain an accurate filtered, back-propagated solution. In this approach two components of the measured field yield the six components of the field closer to the source. Of particular interest is the active part of the Poynting vector that is constructed from the back-projected fields, providing the power per unit area radiated from the sources. We believe it may be of immense practical use in diagnosis of electromagnetic sources underwater. We test the theory with a numerical experiment using a linear array of either magnetic or electric dipole sources excited in a frequency range of 1 to 1000 Hz in seawater that generate two cylindrical holograms (30m radius) of the axial polarization of the magnetic and electric fields, respectively. The complete (all polarizations) electric and magnetic fields are predicted along with the real and imaginary parts of the Poynting vector on a cylindrical back-propagation surface (20m radius). These simulations show that very accurate results are obtained even with low signal-to-noise levels. Work supported by the Office of Naval Research.

NONLINEAR CALIBRATION OF POLARIMETRIC RADAR CROSS SECTION SYSTEMS
Lorant Muth, November 2009

Polarimetric radar cross section systems are charac­terized by polarimetric system parameters Eh and Ev. These parameters can be measured with the use of rotating dihedrals. The full polarimetric dataset as a function of the angle of rotation can be analyzed with a nonlinear set of calibration equations to yield the system-parameter complex constants and the four po­larimetric calibration amplitudes. These amplitudes appropriately reproduce the system drift and satisfy a drift-free system con.guration criterion very accu­rately. The results indicate that the nonlinear ap­proach is better than the previously studied linear ap­proach, which yielded system parameters that are se­riously distorted by drift.

An optimized approach to plane wave synthesis
Amedeo Capozzoli (Università degli Studi di Napoli Federico II),C. Curcio (Università degli Studi di Napoli Federico II), G. D’Elia (Università degli Studi di Napoli Federico II), A. De Simone (Università degli Studi di Napoli Federico II), A. Liseno (Università degli Studi di Napoli Federico II), November 2008

A plane wave generator [1] is of interest in many applications such as the testing on complex and large structures, f.i. the testing of the aircraft-integrated electronic components, or in the characterization of large antennas by means of Compact Antenna Test Ranges (CATR). In these applications it is required to generate a plane wave field in a prescribed volume V in the near zone of the source, disregarding the field behaviour outside V. Concerning the first application, new ideas have been recently introduced in [1] by considering a (highly integrated) Plane Wave Synthesizer (PWS), wherein arrays of radiators are used as electromagnetic sources and are optimized to generate the required plane wave field in a prescribed region. On the other side, standard CATRs, usually made by means of large and expensive reflectors or lens, can be substituted by suitably designed PWS to avoid their main drawbacks caused by diffraction, blocking effects and mechanical tolerances.

CONSIDERATIONS FOR VHF GAIN MEASUREMENTS OVER SEAWATER
John P. Casey (Naval Undersea Warfare Center),Stephen M. Davis (Naval Undersea Warfare Center), Bruce Greenhalgh (Naval Undersea Warfare Center), Rodney P. Gudz (Naval Undersea Warfare Center), Paul M. Mileski (Naval Undersea Warfare Center), Paul Medeiros (Naval Undersea Warfare Center), David A. Tonn (Naval Undersea Warfare Center), Hailu M. Waka (Naval Undersea Warfare Center), Isaac M. Wheeler (Naval Undersea Warfare Center), November 2008

Methods for measuring the gain of an antenna at low frequencies (i.e. below 30 MHz) operating at or near the surface of the ocean on an open-air antenna range have been considered and reported previously. These methods employ a groundwave correction approach and a virtual reference antenna consisting of an idealized quarter wave monopole. However, this approach is not appropriate for application at the shorter wavelengths that occur in the VHF band owing to a variety of factors. In this paper, we shall discuss some of the issues associated with the use of the more conventional substitution method for measuring the gain of an unknown antenna that is operating near the air-sea interface. Issues and challenges related to instrumentation, antenna siting, range assessment, multipath effects, and reference measurements shall be considered.

A Measurement Setup for Characterizing Antenna on an Infinite Ground Plane from 1 to 18 GHz
Justin Kasemodel (The Ohio State University),Chi-Chih Chen (The Ohio State University), November 2008

Currently there is a lack of facilities capable of measuring the full upper hemisphere radiation patterns of antennas mounted on an infinite ground plane. Measurements performed with a finite ground plane suffer diffraction interference from the truncated edges. To circumvent this problem, a new measurement setup was developed at the Ohio State University ElectroScience Laboratory (ESL) for fully characterizing upper hemisphere radiation gain patterns and polarization for antennas up to 4” in diameter from 1-18 GHz. A probe antenna is positioned 46” away from the antenna under test (AUT). The ground plane end diffractions are removed using time-domain gating. The key design consideration is to position the probe antenna in the far-field region and yet shorter than the radius of the ground plane. This paper will present the calibration procedure necessary for the measurement system and it’s limitations due to ground plane probe antenna coupling at low elevation angles. In addition, the complete radiation pattern of a 4” monopole measured from 1-5.5GHz to demonstrate the systems capability for the lower third of the systems operating frequency range.

An Ethernet Based Controller for Managing Complex Antenna Measurements with a Vector Network Analyzer
Marion Baggett (MI Technologies),James Langston (MI Technologies), November 2008

Vector Network Analyzers (VNA’s) are finding increasing utilization in antenna measurement ranges. At the same time, complex measurement scenarios involving many data channels in the antenna under test along with integration to beam steering computers for phased array antennas require management of the data collection beyond the VNA. Traditional methods have added control cards in the measurement control computer, increasing software complexity and reducing measurement throughput. The MI-788 Networked Acquisition Controller is designed to manage the hardware handshakes between position controllers, external sources and VNA’s, control up to 16 channels of multiplexed data from the antenna under test and/or interface with a beam steering computer. The MI-788 tremendously increases system throughput, particularly in these more complex measurement scenarios by removing real time data collection responsibilities from the measurement control computer. In addition, this unit makes all instrument communication Ethernet based, eliminating the spacing and operational limitations of GPIB based measurement systems. This paper will describe the operation of the MI-788 and demonstrate the increased measurement capabilities while using VNA’s in antenna measurements.

Free Space VSWR Method for Anechoic Chamber Electromagnetic Performance Evaluation
Brian B. Tian (MI Technologies), November 2008

This paper gives a detailed account of free space Voltage Standing Wave Ratio (VSWR) method. We first review the formulations and terms commonly used in this method. We then discuss errors involved in its direction determination of extraneous signals, contrasting them among plane wave, spherical wave and specular reflection. We highlight issues relating to its application in anechoic chamber electromagnetic performance. Also discussed is the practice of data processing through analyzing a measured VSWR pattern.

Coordination and Networking of Antenna Research in Europe: Past, Present and Future
Juan R. Mosig (Electromagnetics and Acoustics Laboratory), November 2008

In the last ten years, coordination and networking of Antenna Research in Europe have been greatly influenced by the scientific strategy and policies of the European Union and in particular by the recent "Framework Programmes" FP6 and FP7, launched by the European Commission. Doubtless, there are still relevant R&D programmes at the national level and, obviously, the key European industries and companies continue to perform internally funded (and frequently confidential) research and development on antennas. But there is a clear trend now to put together the expertises of the different actors from all the European countries (industry and academia) and perform mid-term or long-term research within large international groups. The Networks of Excellence (NoE) and the European Cooperation in the field of Scientific and Technical Research (COST) are two of these European Research funding instruments and the European Antenna Community has managed to be present in both.

Antenna Measurement Activities in the European COST project IC0603 ASSIST
Fernando Las-Heras (COST IC603 ASSIST),Olav Breinbjerg (COST IC603 ASSIST), November 2008

This paper describes some of the activities developed in the Working Group 4: “Measurements and Characterization, Technological Issues” of the European COST Action IC603 ASSIST (‘Antenna Systems and Sensors for Information Society Technologies’) in the frame of European research cooperation during its first year of working. During this time COST IC0603 – WG4 has been the unique framework for European research cooperation in the field of Antenna Measurement from the ending of EU Network of Excellence ACE (Antenna Centre of Excellence) until the beginning of the professional organization EurAAP (European Association of Antennas and Propagation). The paper outlines the activities of such WG4, mainly those presented in the COST IC0603 Workshops held up to now.

Problems Using Network Analyzers for Certain Antenna and Radome Measurements
Henry Burger (NAVAIR), November 2008

Prior to modern computer-aided measurement techniques all measurements were made with analog procedures that required the personal attention of measurement professionals. Modern techniques rely on careful set-up involving standards, and the test equipment applies error correction based on these standards. How-ever, there is an over-reliance on computer-based measurement equipment to do all of the thinking, leading to inappropriate use of these techniques, in turn leading to large, unsuspected measurement errors. This paper analyzes a situation wherein a network analyzer was used to isolate radome insertion loss from a combination measurement. The procedure used led to gross errors indicating gain in a passive device. The source of these errors is identified as an incorrect referencing procedure used to isolate radome characteristics from the combined antenna-radome characteristics. This error is common and applies to an entire class of measurement problems.

Cost Effective Extension of Antenna Measurement and Calibration Capabilities up to 80 GHz using a 40 GHz Vector Network Analyzer
Thomas Kleine-Ostmann (Physikalisch-Technische Bundesanstalt),Thorsten Schrader (Physikalisch-Technische Bundesanstalt), Vince Rodriguez (ETS-Lindgren), Zhong Chen (ETS-Lindgren), November 2008

The extension of the frequency range for commercial applications of mm-waves to 80 GHz and beyond often requires extended antenna characterization capabilities both at manufacturer and end-user facilities. Presently, most measurements are based on direct measurements using vector network analyzers (VNAs). VNAs that cover a continuous frequency range up to 67 GHz are commercially available. Above 50 GHz, extensions based on external mixers in waveguide technology are typically utilized. They require a tunable local oscillator (LO) that is usually provided by the two additional ports of a 4-port VNA. However, these extensions not only are restricted in bandwidth but also require a significant financial investment especially considering the fact that the expensive 4-port instrumentation is needed. As most laboratories already have conventional 2-port VNAs usable up to 10 GHz or higher and most antenna characterizations are based on transmission measurements, we present a simple extension scheme based on external mixers and a fixed frequency LO that allows for transmission factor measurements. We demonstrate the feasibility of such an extension scheme for transmissions between a pair of horn antennas ranging up to 60 GHz. The measurements include variation of antenna spacing and steering angle and are verified with a computational analysis based on the finite differences time-domain (FDTD) method.

DIGITAL NOISE RADAR PROTOTYPE DEVELOPMENT
ERIC WALTON (Ohio State University), November 2008

The Noise Radar (actually an ultra-wide band (UWB) spread spectrum radar) is a radar that generates a very wide band pseudo-random waveform and (optionally) up-converts the waveform to a desired microwave frequency spectrum. The bandwidth may be more than 5 GHz. The digital system generates a pair of GHz bandwidth pseudo-random waveforms. The two waveforms may either be identical or the pair of waveforms can be specially designed to be matched to the radar target and its environment. The first waveform is transmitted without a carrier, or it may be up-converted to a high microwave frequency. On receive; the second waveform is cross correlated with the received signals. Specific design of the two waveforms is possible so that the cross correlation coefficient forms an optimized peak for a particular target or class of targets, or to maximize the difference in the response between clutter and targets of interest. A design where a multi-GHz waveform is generated using a FIFO chip and a serializer chip will be developed. Construction of this radar and sample data will be shown. Detection range versus Doppler images will be presented.

Quiet Zone Field Probing using an Inverted Stewert Platform and a Precision Sphere
Alan Buterbaugh (Air Force Research Laboratory),Brian M. Kent (Air Force Research Laboratory), Byron Welsh (Air Force Research Laboratory), November 2008

This paper presents the initial field probe characterization results for an RF scattering compact range using a high precision calibration sphere. This approach uses an Inverted Stewart Platform to position the ultra-sphere through the target quite zone. The Inverted Stewert Platform and optical target tracking system provide a fast and efficient for performing a volumetric incident illumination field characterization of the compact range quite zone using a backscatter RF measurement. The Inverted Stewert Platform system uses six small diameter strings attached to the ultra-sphere to provide the ultra-sphere positioning over the entire quiet zone of the compact range. The inverted Stewart platform also offers increased stability of the target by damping out the torsional pendulum motion typically encountered in conventional string support systems. This presentation will discuss an in-house development of the sphere field probe and discuss advantages and disadvantages of the ultra-sphere volumetric field probe.

RCS Measurements at 320 GHz to Verify the Alignment of the PLANCK Reflector Configuration.
Maurice Paquay (ESA-ESTEC),Bruno Maffei (University of Manchester), Denis Dubruel (Thales Alenia Space), Dominic Doyle (ESA-ESTEC), Gerald Crone (ESA-ESTEC), Gilbert Forma (Thales Alenia Space), Javier Marti-Canales (ESA-ESTEC), Richard Hills (University of Cambridge), Richard Wylde (Thomas Keating Ltd.), Luis Rolo (ESA-ESTEC), Jan Tauber (ESA-ESTEC), November 2008

In the Flight Model (FM) of the PLANCK telescope, the feed horns are connected to either HEMTs or bolometers operating at cryogenic temperatures to detect the Cosmic Microwave Background radiometric signal. For the purpose of an overall alignment verification at ambient temperature, RCS measurements have been performed using an auxiliary feed horn that is terminated with a switching diode. This verification test has been conducted at 320 GHz, to benefit from the narrow beam and a high sensitivity to misalignment. To perform the RCS measurements, an additional “circulator” with low propagation loss and high isolation from transmit to return channel had to be developed. Besides that, the circulator also co-locates the phase centres of both Tx and Rx range antennas on the focal point of the CATR, which allows mono-static RCS measurements. Quasi-optical techniques have been used to design a circulator that meets these requirements. To test the feasibility of determining the feed location from the RCS measurements with an uncertainty of ±1 mm, a test campaign was conducted with the so called RF Qualification Model (RFQM). In this campaign, 9 feed locations with 1 mm separation were tested. With the Flight Model, the test was on the critical path of the planning and only one test could be conducted to verify the overall alignment.

Polarimetric calibration of indoor and outdoor polarimetric radar cross section systems
L.A. Muth (National Institute of Standards and Technology), November 2008

We used a set of dihedrals to perform polarimetric calibrations on an indoor RCS measurement range. We obtain simultaneously hh, hv, vh, and vv polarimetric data as the calibration dihedrals rotate about the line-of-sight to the radar. We applied Fourier analysis to the data to determine the polarimetric system parameters, which are expected to be very small. We also obtained polarimetric measurements on two cylinders to verify the accuracy of the system parameters. We developed simple criteria to assess the data consistency over the very large dynamic range demanded by the dihedrals. We examined data contamination by system drift, dynamic range nonlinearities, and the presence of background and noise. We propose improved measurement procedures to enhance consistency between the dihedral and cylinder measurements and to minimize the uncertainty in the polarimetric system parameters. The final recommened procedure can be used to calibrate polarimetrically both indoor and outdoor ranges.

Sharp extraction of energy of bright points of a target
Renaud Cariou (Radar Cross Section Department), November 2008

At the present time at the end of a measurement of RCS of a target, it is possible to obtain either the value of the RCS of the target as a whole for a given frequency, bearing and elevation or a RADAR image of this target. The aim of this RADAR image is generally to locate the bright points that constitute the target and not to estimate the energy of these bright points. That is why the calculation of these energies is not generally the subject of an elaborate rigorous processing. Yet it may be necessary to be able to give the RCS of any part of the target when this target has been measured as a whole. In answer to this need it is necessary to isolate and calculate sharply the energies of the bright points that constitute the target, because the RCS of each part of the target is the sum of the energies of the bright points which constitute it. This article exposes a method of processing which allows this calculation, while resolving the problems linked to the interpolation and to the discrete nature of the measurements and calculations.

WIDE BAND DUAL POLARISED V/UHF PHASED ARRAY FOR INDOOR RCS MEASUREMENT
L.J. Foged (SATIMO Italy),Andrea Giacomini (SATIMO Italy), Philippe Berisset (CEA/CESTA), Roberto Morbidini (SATIMO Italy), Thierry Blin (SATIMO Italy), Yannick CHEVALIER (CEA/CESTA), A. Menard (DGA), November 2008

Phased arrays antennas have desirable features in terms of simplicity, compact dimensions and low weight for low frequency applications requiring dual polarization and medium gain such as RCS measurements. However, a fundamental problem with phased arrays technology in wide band applications is grating lobe limitations due to the grid topology of the phased array elements. The spacing of the array elements cannot be to close in order to limit element coupling and not to large to avoid grating lobes. Consequently, conventional phase array antenna applications are generally limited to a useable frequency bandwidth of 1:2. A unique grid topology has recently been developed to overcome this problem [1, 2]. By interleaving three separate phased arrays, each dedicated to a different subband with close to 1:2 bandwidth, the useable bandwidth of the combined phased array antenna can be extended to as much as 1:7 while maintaining the nice performance features of the basic phase array technology. Based on this technology a large dual polarized phase array antenna has been designed for indoor RCS testing in the frequency range from 140MHz to 1000MHz. The operational bandwidth of the array is split into three subbands: 140-260 MHz, 260-520 MHz and 520-1000 MHz. The array is 6.34 x 6m and weighs less than 250Kg. Due to the element spacing and topology the phased array is sensitive to excitation errors so the beam forming network (BFN) feeding the elements must be wellbalanced. A uniform amplitude and phase distribution for the array excitation coefficients has been selected to simplify the BFN design and minimize possible excitation errors throughout the bandwidth. This paper describe the antenna electrical design and performance trade-off activity, the manufacturing details and discuss the comprehensive validation/testing activity prior to delivery to the final customer.

Cross Polarization Uncertainty in Near-Field Probe Correction
Allen Newell (Nearfield Systems Inc.), November 2008

The probe correction of near-field measured data can be considered as being composed of two parts. The first part is a pattern correction that corrects for the effects of the aperture size and shape of the probe and can be analyzed in terms of the far-field main component pattern of the probe. The second part is due to the non-ideal polarization properties of the probe. If the probe responded to only one vector component of the incident field in all directions, this correction would be unnecessary. But since all probes have some response to each of two orthogonal components, the polarization correction must be included. The polarization correction will be the focus of the following discussion. Previous studies have derived and tested general equations to analyze polarization uncertainty12. This paper simplifies these equations for easier application. The results of analysis and measurements for Planar, Cylindrical and Spherical near-field measurements will be summarized in a form that is general, easily applied and useful. Equations and graphs will be presented that can be used to estimate the uncertainty in the polarization correction for different AUT/Probe polarization combinations and measurement geometries. The planar case will be considered first where the concepts are derived from the probe correction theory and computer simulation and then extended to the other measurement geometries.

Spherical Near-Field Antenna Measurement Note: Insertion Loss Gain Measurement
Brian B. Tian (MI Technologies), November 2008

This note highlights the connection of antenna gain to the measurement of insertion loss based on established SNF formulations, relating directly among antenna transmission coefficients, antenna gain, acquired SNF raw scan data and the parameters acquired during a range insertion loss measurement. It shows how the measured insertion loss parameters are applied in normalizing raw SNF scan data in determining antenna gain.

CIRCULAR POLARISED WIDE BAND FIELD PROBES
L.J. Foged (SATIMO Italy) ,Andrea Giacomini (SATIMO Italy), H.C. Sanadiya (Indian Space Research Organisation), R.K. Malaviya (Indian Space Research Organisation), Roberto Morbidini (SATIMO Italy ), S.B. Sharma (Indian Space Research Organisation), Viren R. Sheth (Indian Space Research Organisation), November 2008

Dual polarized probes for modern high precision measurement systems have strict requirements in terms of pattern shape, polarization purity, return loss and port-to-port isolation. A desired feature of a good probe is that the useable bandwidth should exceed that of the antenna under test so that probe mounting and alignment is performed only once during a measurement campaign. As a consequence, the probe design is a trade-off between performance requirements and the usable bandwidth of the probe. For measurement applications in circular polarization the choice is between measuring the linear polarization components separately and derive the resulting circular polarized by computation or to measure directly with a circular polarized probe. Dual polarized probes in circular polarization with high polarization purity is difficult to achieve on a wide bandwidth. Dual linear polarized probe technology has recently been developed capable of achieving as much as 1:4 bandwidth while maintaining the high performance of traditional probe designs [1–7]. This paper describes the development, manufacturing and test of dual circular polarized probes with as much as 1:2 bandwidth as shown in Figure 1.







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